1. Field of the Invention
The invention relates to a method for the operation of a converter, in particular of a wind power plant, of a power-generating generator system, in particular of a wind power plant, that can be connected to an electrical AC network and the use of a converter. The invention also relates to a computer program product with program code means, a data carrier, a power-generating generator system and a wind power plant.
2. Description of Related Art
Electrical AC networks are normally operated with a base frequency of approx. 50 Hz or 60 Hz in North America. Electrical power generators, for example power plants, generators or wind power plants, supply the generated power to the AC network at this base frequency. Small or large electrical consumers, such as electrical household devices or industrial companies, take the required power from the AC network in the form of AC power.
AC networks are also used to transfer and control signals. For example, signals are supplied to the network and are transmitted to signal receivers by means of so-called audio frequency ripple control systems. “Audio frequency” (also “AF”) refers to the frequency range from approx. 100 Hz to approx. 500 Hz, which is also relayed in an only insignificantly weakened state in the AC network over greater distances. Typically, each ripple control system transmits at a ripple control frequency determined by the network operator in the audio frequency range.
The ripple control signals are superimposed on the power supply voltage at the connection point of a ripple control system as additional control or AC voltage. The amplitude of the control voltage is thereby in the range of less than 1 percent to a few percent of the amplitude of the base frequency in the AC network. In this connection, the terms “impressed,” “added up,” and “superimposed” are used synonymously.
En route to the receiver of the ripple control signal, the signal is weakened by the consumer and power generator connected to the network in that the connected consumers or generators take part of the signal current and, thus, reduce the remaining signal current in the AC network. The weakening is undesired, since the signal levels themselves are already small. In order to keep the weakening as small as possible, the least possible, or no signal current, should be taken from the consumers or generators connected to the AC network.
The amount of signal current taken by a generator or a consumer depends on the signal voltage and the impedance of the respective consumer or generator at the ripple control frequency. The impedance is the frequency-dependent (complex) resistance of the consumer or generator. The higher the real portion of the impedance of the consumer or generator at the ripple control frequency, the less signal power is absorbed by the consumer or generator. In order to counteract a weakening of ripple control signals in the AC network, a high impedance of the generator or consumer is thus required at the ripple control frequency.
The following covers the situation with power generators, in particular wind power plants. The same considerations also apply to other power generators.
Wind power plants, the electrical components of which include generator, frequency converter and transformer, generally already have sufficient impedance at the ripple control frequency so that additional means do not need to be used to obtain ripple control signals in the AC network. However, this is not sufficient at some locations, in particular when small signal levels are used.
A slight impedance increase is possible with passive means. For an impedance increase of 10 to 20 percent, it is sufficient to replace an already existing transformer with a transformer with a higher short-circuit voltage value or to switch the voltage taps, if possible. For a stronger increase in the impedance at the audio frequency, a passive audio frequency block or a passive filter can also be used in the form of capacitor banks.
Both the transformer, with a higher short-circuit voltage value, and the passive audio frequency block have a higher required space due to the larger dimensioned transformer or due to the necessity of large-dimensioned capacitor banks. These measures also cause increased losses and additional costs. The achievable impedance increase is limited.
A strong improvement is achieved with an active audio frequency block. It works such that audio frequency currents, i.e. AC currents at the ripple control frequency or audio frequency, are first let into the system unfiltered, together with the base frequency from the AC network. The entire current flowing into the system is measured in a total current measurement. The share of the audio frequency current or the signal current is determined by means of a frequency filter set to the audio frequency or the ripple control frequency.
The measured audio frequency current is fed to an amplifier, which generates an audio frequency compensation current suitable for the audio frequency current, which is fed back into the network in front of the measuring point. The sum of the audio frequency current flowing into the system and the audio frequency compensation current fed back into the network cancel each other out so that the overall arrangement for the audio frequency appears to have a very high impedance. The arrangement is thus very effective, but is expensive due to the large amplifier and space required. Even with this technique, losses in the supplied power are unavoidable.
In addition to the ripple control signals, other generators or consumers also generate audio frequency currents in the AC network, above all harmonic oscillations of the base frequency of the AC network, but also other audio frequency shares.